This section provides background information related to the present disclosure which is not necessarily prior art.
High quality class-A surfaces made using composite sheet molding compound (SMC) panels are typically joined to a support structure using polymeric based thermoset adhesives prior to attaching to the vehicle bodies. For example, exterior SMC door panels are often bonded to an internal reinforcing member by a thermoset adhesive. Typically, adhesive-bonding requires a thermal cure cycle in order to facilitate curing and to produce a strong joint. However, the thermal cure cycle intended to cure the adhesive introduces minute surface distortions in the vicinity of the bond-line known as bond-line read-out (BLRO) which deteriorates the visual appeal of the class-A surface. Significant BLRO often results in part rejection and the subsequent time and cost of rework. There remains a need for a cost effective method and apparatus to mitigate the severity of BLRO defects resulting from the curing of adhesives.
In the automotive practices, a polymeric-based thermoset adhesive is cured using a two-stage process. In the first stage, uncured adhesive is applied between the substrates to be joined. External pressure is then applied upon a bonding fixture to maintain dimensional tolerances as it is subjected to the cure temperature for 3-5 minutes. Heat of about 1 kWh at 10 kW is supplied to the adhesive by conduction via the fixture and the substrate until the adhesive is cured to “green strength,” which allows one to handle panels joined by adhesive before the adhesive is completely cured. “Green strength” allows joined panels to continue through assembly while the adhesive continues to cure. Since the heat flow to the adhesive from the fixture occurs through the panels, they experience an overall rise in temperature that results in thermal expansion. This, in combination with any differences in the coefficients of linear thermal expansion (CLTEs) between the panels, leads to distortion of the bond-line. At the same time, the adhesive itself marginally distorts due to chemical cure shrinkage. As the adhesive cures partially to green strength, a fraction of these distortions are retained in the joint. The panels joined to green strength are then painted in a paint booth followed by additional heating to cure the paint. The paint “bake” cycle requires another 20-30 minutes of heating of the assembly, resulting in further surface distortion. This process takes a total of about 40 minutes at about 31 kWh per part.
The unwanted side effects of curing adhesives with heat in current practices include the waste of energy spent in the undesired heating of the substrates and surface distortion due to SMC thermal expansion and dimensional changes, or BLRO. There remains a need for a cost-effective method and apparatus to mitigate the severity of BLRO defects resulting from the curing of adhesives.